In Reply.—Dr Weston's concern is appropriate. In the review by Greene,1 the derived 75% proportion is inflated due to his misinterpretation of a previous report.2 The 37% incidence in our study is similar to that in previous reports from Seattle (39% and 42%)3,4 and is comparable to the experience that Dr Weston describes.5 The 63% proportion reported by Eisenberg and associates6 from suburban King County described findings in selected patients. More recent data from that emergency medical services system indicate that ventricular fibrillation was present in 37.6% of several thousand consecutive arrests, excluding trauma-related events (Mickey S. Eisenberg, MD, PhD, Richard O. Cummins, MD, and Mary Pat Larsen, MA, written communication, December 1990). Hence, 35% to 40% is a reasonable approximation of the proportion of out-of-hospital arrest victims who have ventricular fibrillation as the first recorded rhythm.The figure will differ depending on the